This invention relates to an optical apparatus for use as an endoscope or borescope for inspection of inaccessible objects.
It is desirable for such apparatus to be provided with variable magnification in the form of a zoom lens which ideally should be located at the distal end of the instrument and may incorporate the objective lens. Endoscopes and borescopes however typically comprise a long tube of the order of 10 mm or less in diameter. Consequently any mechanism for providing independent movement of lenses within the zoom lens must be miniature in form and must be actuated by a mechanism which does not significantly obstruct the available space for a viewing means such as an optical relay which must extend through the tube.
A particular difficulty encountered in the incorporation of such zoom lenses in such apparatus is that of providing compensation for movement of the focal plane of the zoom lens when magnification is varied. In the absence of compensation the viewed image may be completely out of focus and indistinguishable after each adjustment of magnification and it is undesirable for the user to have to carry out anything other than a minor re-focusing operation after each adjustment.